Recently, system products using millimeter wave band such as 24 GHz and 77 GHz car radar, 60 GHz radio near field communication and 94 GHz RF-imaging are being actively researched.
Such system products using millimeter wave band are configured by combining various types of individual circuits. As millimeter wave band circuit, a single end type circuit is generally used. However, in case of a mixer circuit, a “Gilbert Cell” type circuit which is operated in a differential mode is mainly used due to advantage that LO-IF interference and even-order distortion are reduced.
According to recent research on an amplifier circuit having a high operation frequency, a virtual ground can be utilized, and a differential mode amplifier capable of improving noise characteristics compared with a single mode amplifier, is being much utilized.
As a single mode circuit is generally used, a balun circuit for effectively converting between signals of two modes is required in the entire system.
Due to enhanced performance of an active device together with development of semiconductor processes, operation frequency of millimeter wave band products gradually shifts to a high frequency band. At a high frequency band, degree of integration of products can be enhanced due to decrease in size of a passive circuit. However, compared with a case of a low frequency band, problem such as change in circuit performance due to loss increase and a process error may occur.
As a method for converting a signal into a differential mode using a single-end feed, a transformer may be used at a low frequency band of 3 GHz or less. The transformer, which uses a coil type inductor therein as an integrated circuit (IC), has a problem that great loss occurs at millimeter wave band.
A circuit such as a Marchand balun or a Rat Race is mainly used for conversion between a single mode signal and a differential mode signal at millimeter wave band.
FIG. 1 shows a view illustrating a circuit diagram (a) of a Marchand balun used to form a differential mode signal, and a Marchand balun substantially fabricated in an integrated circuit (IC) chip in accordance with the conventional art.
As shown in FIG. 1, the Marchand balun is implemented by using coupling of two transmission lines having ¼ wavelength, and the Rat Race is also implemented by using long transmission lines having ¾ wavelength.
Such circuits may occupy a large area in an IC chip, and may cause great loss at a high frequency of millimeter wave band. Especially, the Marchand balun is frequently used in designing an IC, due to broad bandwidth characteristics. However, the Marchand balun may cause great change in characteristics due to process error, because it is sensitive to coupling change between two transmission lines.